Medium wave - back to the roots

Motivation:

Receiving:

Back in the early 90s I used an active indoor antenna for medium and short wave reception, which worked quite well.

Pretty fast I discovered that in today's environment I could only receive QRM that way.
There are a lot more "electronic devices" around in operation in a typical german household these days, compared to the pre-Internet era.

So far my optimum receive coverage could be achieved using

- a PA0RDT mini-whip receive antenna.
  There are several kits available in the Internet, even fully assembled antenna and power-feed units.
  But you may also set up the PCB yourself by milling a single-sided copper-plated board.

- a standalone SDR receiver solution, the so-called ARM Radio.
  It is based on an Micro-controller evaluation board with display and touch-screen.


So far it shows great performance at a low price (< 40 USD) and it is superior to e.g. my good old Alinco DX-77 TRX.

The image below shows the receive coverage (630 m band, WSPR) using the ARM Radio with the mini-whip.




A second plausibility check for your receive rig are none-directional beacons ( NDB ).
These beacons - used for aeronautical direction finding - transmit between 190 kHz and 535 kHz.
They may be used for propagation experiments as well.

Here is an overview of the NDBs that I can receive at my location (just a quick band scan):

Transmitting QRPp:

As a decent engineer, It took me months of brain-storming, simulation and several emails exchanged with Stefan, DK7FC.
Finally, I decided on setting up a small vertical loop.

The loop has a trapezoid shape with ca. 16 m circumference.
It is made up of 13 parallel braided copper wires of 0.5 mm² each (so it is a loop with 1 winding).
Resonance is achieved using ca. 5 nF in parallel to the open loop.
With an additional 1:13 balun I could transform the loop impedance up to ca. 50 Ohm for coax feeding.

  

Using the Magnetic loop calculator by DG0KW, I calculated the loop "gain" as -38 dBi.

When I had the antenna carefully tuned to the WSPR TX frequency of 476 kHz, the next important question came up:
Which transmitter to use ?

OK, there is my proven Ultimate 2 with 200 mW output, so far used "barefoot" on 21 MHz and 28 MHz.

It does support 475 kHz operation as well, putting ca. 200 mW out.
But 200 mW into a -38 dBi antenna:  That is is real QRP (or better "QRPp")

With 200 mW (23 dBm) TX output and the above simulated antenna gain,
I have calculated an equivalent isotropical radiated power (EIRP) of ca. -20 dBm EIRP (= 10 µW EIRP)

10 µW of radiated power, that seemed to me too much QRPp, even using WSPR.
Honestly speaking, I did not expect too much WSPR spots with this setup.

This is the result after 6 weeks of operation using 10 µW EIRP (200 mW into the 16 m loop) on 475 kHz:



Mileage highscore:

Upgrade from QRPp to QRP:

In the next step I set up a 25 W amplifier and later on a 40 W amplifier.
The image below shows the first iteration step of the 40 W variant :



With that amplifier I have gained ca. 23 dB.
So the estimated output is now ca. 7 dBm EIRP (5 mW of radiated power):

The extra power is a real improvement: